1. Field of the Invention
The present invention relates to an automation or control network such as a fieldbus network that facilitates an extended level of redundancy such as redundancy within the power distribution facilities of a fieldbus network.
2. Description of the Related Art
Fieldbus networks are advantageously used in industrial control systems and particularly in industrial control systems that provide distributed control incorporating field devices having local processors. A fieldbus network is a two-wire network capable of delivering DC power to remotely installed field devices and capable of providing bidirectional digital communications between the remote field devices and the host system. The digital communications are typically carried on a 31.25 kHz carrier signal in the standard-defined H1 fieldbus network. Various field devices might be attached to be powered from and communicate over the network, including controllers, actuators and sensors. Multiple field devices can be attached to one fieldbus segment. The fieldbus is typically implemented on the physical level as two-wire shielded cable. More details about conventional aspects and implementations of fieldbus networks can be found in the International Electrotechnical Commission standard IEC 61158-2 which is specifically for industrial networks and promoted by the FOUNDATION Fieldbus and PROFIBUS organizations.
On the host system side of the fieldbus network, primary and backup H1 cards are mounted as front end interface modules to the networked field devices to provide continuous communications. The backup H1 card provides redundancy and takes over the communications between the host and the field devices in case the primary H1 card fails due to any reason. Each H1 card might be configured to provide more than one channel or more than one segment per card, depending on the manufacturer.
For simplicity, the following discussion refers to one fieldbus segment per H1 card, but implementations can readily provide more segments per card. In a conventional system, the output connection of the primary and the backup H1 cards can be linked together at the host system so that one single two-wire cable can be interfaced directly between the conditioned power modules and the H1 cards. In some other implementations, both H1 cards can be wired individually to the conditioned power modules so that the common link between the cables is made at the power modules. The latter method is used to provide an additional cable redundancy between the H1 cards and the conditioned power modules.
The conditioned power modules also provide DC power to remote field devices via a single two-wire cable connected to an electronic device coupler. Some types of conditioned power modules are designed with a redundancy feature in a fashion similar to H1 cards. When two conditioned power supplies with this redundancy feature are connected in parallel to the networked field devices, both power supply modules operate together and share the same load. If either of the power supply modules fails, the remaining, healthy power supply module will supply the extra power to the load (i.e., the field devices).
In a fieldbus network, the communication signals should be terminated at both ends of the network cable. A matching terminator circuit is fitted at the front end of the power module, and another matching terminator circuit is mounted at the last node of the network. A typical network may have multiple drop or spur connections that each interface to one or more local field devices.
FIG. 1 is a schematic circuit diagram that illustrates the common method of installation recommended by the IEC 61158-2 standard. FIG. 1 shows a fieldbus network 10 in which all field devices D1, Dn (n could be up to 32 devices per segment) are linked to the host through primary H1 card 12 and backup H1 card 14 and through the single cable 16. Cable 16 and the communication signals are terminated at both ends of the cable network by terminators T1 and T2. Primary conditioned power supply module 18 and secondary conditioned power supply module 20 are connected in parallel to bus 16 so as to share the load of the field devices D1, Dn. As illustrated, field devices may be connected to the network cable 16 through a device coupler 22. Typical device couplers 22 provide a standardized interface that allows for easier connection of spurs or individual field devices to the network cable 16. Some device couplers provide fuses or current limiting technology to address local faults and to provide local fault indicators. One of the restricting factors in FOUNDATION fieldbus technology is that the physical layer used for the H1 network does not naturally allow for redundancy. This lack of redundancy may in part lead to undesirable expense and down time for the fieldbus network.